A cutting tool used for a metal machining process (e.g., milling) generally comprises a tool body and a plurality of cutting inserts mounted thereto. A cutting insert is either directly mounted to a tool body or indirectly connected to the same using a cartridge. The cutting insert comprises cutting edges that contact a workpiece so as to cut it during a cutting process.
Cutting inserts having a top surface, a bottom surface and side surfaces connecting the top and bottom surfaces have been known. The side surfaces form an obtuse angle with the bottom surface as well as an acute angle with the top surface. The cutting inserts have cutting edges formed at an intersection between the top surface and the side surfaces. The cutting inserts are one-sided with cutting edges provided only on its top surface. They are advantageous in terms of preventing interference between a side surface of the cutting insert and a machined surface during milling, and provide a sharp cutting edge.
Korean Patent Publication No. 10-2008-0107315 discloses such a prior cutting insert, which is shown in FIGS. 1-5. As shown in FIGS. 1 and 2, a cutting insert (10) comprises a top surface (12), a bottom surface (14) and side surfaces (16) connecting the top surface (12) and the bottom surface (14). The side surfaces (16) form an obtuse angle with the bottom surface (14) as well as an acute angle with the top surface (12). Four main cutting edges (18) and four minor cutting edges (20) are formed at an intersection between the top surface (12) and the side surfaces (16). The cutting insert (10) is mounted such that the main cutting edge (18) has a positive axial rake angle as well as a negative radial rake angle.
As shown in FIG. 3, when the cutting insert (10) is mounted in the cutting tool, a upright wall (32) is milled by the perpendicularly disposed main cutting edge (18a). Further, the surface of a base wall (34) is smoothened by the transversely disposed minor cutting edge (20b). As shown in FIG. 2, the minor cutting edge (20) is located closer to the bottom surface (14) of the cutting insert than the main cutting edge (18). Since the side surface (16) forms an obtuse angle with the bottom surface (14), the minor cutting edge (20) situated lower than the main cutting edge (18) is inwardly disposed with respect to the extension of the main cutting edge (18) when the cutting insert (10) is viewed from the above. Accordingly, the minor cutting edge (20a) disposed in the extension of the main cutting edge (18a) is spaced from the upright wall (32), while the main cutting edge (18b) disposed in the extension of the minor cutting edge (20b) is spaced from the base wall (34) and is not involved in cutting. However, it is difficult to form a large positional difference between the cutting edge (18a, 20b) involved in cutting and the cutting edge (18b, 20a), which is not involved in cutting, only by positioning the minor cutting edge (20) to be lower than the main cutting edge (18). Thus, it is difficult to prevent the cutting edge (18b, 20a), which is not involved in cutting, to not contact with the machined surface (34, 32).
Further, since a corner portion (22) of the cutting insert (10) first contacts a workpiece (30) to start cutting during milling, the greatest cutting resistance is generated in the corner portion (22) and the starting portions of the minor cutting edge (20b) and the main cutting edge (18a) connected via the corner portion (22). However, the minor cutting edge (20) is situated lower than the main cutting edge (18) in the prior cutting insert (10). Thus, the cutting insert (10) is thin in the corner portion (22) and the minor cutting edge (20), and does not provide a strength sufficient enough to correspond to a large cutting resistance.
Moreover, FIG. 5 shows that the cutting insert (10) cuts the work piece (30) while it simultaneously moves in the radial and axial directions in relation to the rotational axis of the cutting tool. The direction of motion is indicated by the arrow labeled M. As shown in FIG. 5, the main cutting edge (18 b) also contacts and cuts the base wall (34) during ramp milling. However, the main cutting process is performed by the main cutting edge (18 b). A great cutting resistance is applied to the main cutting edge (18 b). To not weaken the strength of the main cutting edge (18 b), the side surface (16) below the main cutting edge is designed to have a small clearance angle. Accordingly, as shown in FIG. 4, the side surface (16) is almost parallel to or situated nearest to the base wall (34) during the cutting process. When the main cutting edge (18 b) is used in ramp milling, the interference easily occurs between the side surface (16) below the main cutting edge (18 b) and the base wall (34). Thus, it is difficult to perform ramp milling to cut the work piece (30) by moving the prior cutting insert (10) downward and obliquely.